Money item acceptor with enhanced security

ABSTRACT

An acceptor for money items such as coins or banknotes produces a money item parameter signal (x 1 ) depending on a sensed characteristic of the money item. A store ( 12 ) provides window data corresponding to normal acceptance ranges of values of the parameter signal for a money item of a particular denomination (NAW), as well as restricted acceptance windows (RAW). A processor ( 11 ) determines when an occurrence of the parameter signal (x 1 ) may represent a fraudulent money item and then for subsequent sensed money items compares the value of the parameter signals (x 1 ) with the restricted acceptance range (RAW). The RAW range is used until n successive true coins are inserted or a time t has lapsed. After a fraudulent attempt, the values of n and t are increased so that a fraudster cannot then insert n true coins or wait a time t and attempt another fraudulent coin insertion. Also, a focused rejection window (FRW) rejects coins with suspiciously close parameter signals, which could form part of a counterfeit set.

FIELD OF THE INVENTION

This invention relates to an acceptor for money items such as coins andbanknotes and has particular but not exclusive application to amulti-denomination acceptor.

BACKGROUND

Coin and banknote acceptors are well known. One example of a coinacceptor is described in our GB-A-2 169 429. The acceptor includes acoin rundown path along which coins pass through a coin sensing stationat which sensor coils perform a series of inductive tests on the coinsin order to develop coin parameter signals which are indicative of thematerial and metallic content of the coin under test. The coin parametersignals are digitised and compared with stored coin data by means of amicrocontroller to determine the acceptability or otherwise of the testcoin. If the coin is found to be acceptable, the microcontrolleroperates an accept gate so that the coin is directed to an accept path.Otherwise, the accept gate remains inoperative and the coin is directedto a reject path.

In banknote validators, sensors detect characteristics of the banknote.For example, optical detectors can be used to detect the geometricalsize of the banknote, its spectral response to a light source intransmission or reflection, or the presence of magnetic printing ink canbe detected with an appropriate sensor. The parameter signals thusdeveloped are digitised and compared with stored values in a similar wayto the previously described prior art coin acceptor. The acceptabilityof the banknote is determined on the basis of the results of thecomparison.

When a number of coins or banknotes of the same denomination are passedthrough an acceptor, successive values of coin or banknote parameterdata are thus developed. When the distribution of the values of thesesignals is plotted as a graph, the result is a bell curve, with acentral peak and tails on opposite sides. The shape of the graph maytypically although not necessarily be Gaussian.

The distribution illustrates that for a money item, such as a coin orbanknote of a particular denomination, the most probable value of thecorresponding parameter signal lies at the peak of the bell curve, witha decreasing probability to either side. In prior coin and banknotevalidators, data is stored in a memory, corresponding to acceptableranges of parameter signal for a particular denomination. The acceptorthus compares the value for a coin or banknote under test with thestored data to determine authenticity. The data may define windows interms of upper and lower limit values, or as a mean value and a standarddeviation, such that the window comprises a predetermined number ofstandard deviations about the mean. By making the stored windows narrow,an increased discrimination is provided between true money items andfrauds. However, if the windows are made too narrow, the rejection rateof true money items increases, disadvantageously. The width of thewindows is thus selected as a compromise between these two factors.Attempts to defraud coin or banknote validators typically involve themanufacture of facsimile coins or banknotes which cause the acceptor toproduce parameter signals which lie within the stored acceptancewindows.

In U.S. Pat. No. 5,355,989, a coin acceptor is described which switchesfrom using a first normal acceptance window for a true coin, to a secondnarrower window when a coin parameter signal produced by testing a coinfalls in a region of the normal window for the true coin correspondingto a low acceptance probability region for the coin concerned. A groupof fraudulent coins may all have similar characteristics and they maycause the validator to produce parameter signals which lie within thenormal window, but the parameter signals consistently have a value whichis not centred on the high probability peak region of the windowassociated with the true coin but instead are centred on the lowerprobability tail regions of the bell curve distribution within thenormal window. When the parameter signal falls within this lowprobability region, the second narrower window is then used for the nexttested coin. If the next coin has a parameter falling in the narrowerwindow it is a true coin but if not, it is a fraud which should berejected. This approach seeks to prevent frauds carried out by the useof coins of a particular low value denomination, from a foreign currencyset, with characteristics that correspond but are not exactly the sameas a high value coin of the currency set that the acceptor is designedto accept. It will be understood that the foreign denomination coinsexhibit their own generally Gaussian distribution of parameter signals,and if the low probability or tail region of this distribution partiallyoverlaps a corresponding region of the distribution for the true cointhat the acceptor is designed to accept, then the low value foreigncoins will sometimes be accepted as true coins.

However, significant problems are unresolved by U.S. Pat. No. 5,355,989.In the disclosed arrangement, when a true coin is inserted, the systemswitches back from the second narrower window to the first normalacceptance window. If the next coin inserted is a foreign currency coin,if it has a parameter signal within the normal acceptance window, itwill be accepted although the system will then switch to the secondnarrower window for the next coin under test. If the next coin tested isa true coin, it will be accepted and the system will switch back to thefirst window. The US Patent considers the possibility of counting groupsof n coins before making the switch between the windows. Thus, with thissystem, it is possible to obtain acceptance of a significant number offoreign currency coins by alternating them with true coins eitherindividually or in equal numbered groups of n coins. A furtherdisadvantage is that the system is very slow because the foreign coinsdo not all produce an acceptance and so when a fraudster is attemptingto use foreign coins they may be rejected a number of times as a resultof falling outside of the first relatively wide acceptance window.However, the prior validator takes no account of the fraud attempt andwill only respond when a fraudulent coin is in fact accepted.

WO 00/48138 discloses an arrangement to overcome these problems. In oneembodiment, two security barrier ranges are introduced which lie outsidethe normal acceptance window. These security barrier ranges can begenerally aligned with the peak of the distribution for the fraudulentcoin. Even if the fraudulent coin produces a parameter signal outside ofthe normal acceptance window, should the parameter be within thesebarriers, the existence of the fraud attempt is detected, the coin isrejected, and the acceptor switches to the narrower acceptance window toreduce the risk of fraud.

In addition, WO 00/48138 discloses that in the event of a possiblefraudulent attempt, the system is operable to compare any subsequentoccurrences of the parameter signal with the narrower window for apredetermined time and then to revert to the normal acceptance window.Hence merely inserting a set number of true coins directly after aforeign coin will not then result in the system reverting to the normalacceptance window; a certain time must also have elapsed.

In spite of the more complex arrangement disclosed in WO 00/48138, themoney item acceptor described therein has some shortfalls. A perseverantfraudster could make repeated fraudulent attempts and thus determine thenumber of true coins to be inserted or the amount of time to have lapsedbefore the use of the normal acceptance window is resumed. Also,particularly good counterfeit money items could be produced which wheninserted into the money acceptor produce a Gaussian output with a narrowpeak inside even the narrower acceptance window.

SUMMARY OF THE INVENTION

The invention seeks to overcome these problems. In accordance with theinvention from a first aspect there is provided a money item acceptorcomprising: a signal source to produce a money item parameter signal asa function of a sensed characteristic of a money item, a store toprovide data corresponding to a normal acceptance range of values of theparameter signal for a money item of a particular denomination, therange including relatively high and low acceptance probability regionswherein the value of a parameter signal corresponds to a relatively highor low probability of an occurrence of a sensed money item of saidparticular denomination, and a processor configuration operable todetermine when an occurrence of the parameter signal corresponding to afirst money item adopts a predetermined value relationship, and inresponse thereto, to compare the value of a subsequent occurrence of theparameter signal corresponding to a second money item with datacorresponding to a restricted acceptance range as compared with thenormal acceptance range, and to provide an output corresponding toacceptability of the second money item if the second occurrence of theparameter signal falls within said restricted acceptance range, saidprocessor being operable to compare subsequent occurrences of theparameter signal with the restricted acceptance range, and if a firstnumber of them correspond to acceptable money items, to revert to thenormal acceptance range, wherein, the processor is operable afterreverting to the normal acceptance range and in response to a subsequentmoney item parameter signal adopting said predetermined valuerelationship, to compare subsequent occurrences of the parameter signalwith the restricted acceptance range and if a second number of themcorrespond to acceptable money items, to revert to the normal acceptancerange again, the second number being different from the first number.

The money item acceptor may be arranged such that the second number isgreater than the first number, and the processor may be operable toincrement said first number by a predetermined amount to define saidsecond number. Furthermore a counter may be operable to count said firstnumber and thereafter to count said second number, and the processor maybe operable to reset the count counted by the counter to a default countvalue in the event that there is no occurrence of a money item parametersignal within a predetermined security time period.

The predetermined value relationship may occur when an occurrence of themoney item parameter signal has a value within the low acceptanceprobability range or when an occurrence of the money item parametersignal has a value within a predetermined security barrier range outsideof the normal acceptance range.

The processor may be operable to compare occurrences of the money itemparameter signal with said restricted acceptance range for a firstpredetermined time period following an occurrence of the money itemparameter signal that has said predetermined value relationship, andthen to revert to the normal acceptance range and after reverting to thenormal acceptance range to compare occurrences of the money itemparameter signal with said restricted acceptance range for a secondpredetermined time period following an occurrence of the money itemparameter signal adopting said predetermined value relationship, andthen to revert to the normal acceptance range, said second time periodbeing greater than the first time period.

In accordance with the invention from a second aspect there is provideda money item acceptor comprising: a signal source to produce a moneyitem parameter signal as a function of a sensed characteristic of amoney item, a store to provide data corresponding to a normal acceptancerange of values of the parameter signal for a money item of a particulardenomination, the range including relatively high and low acceptanceprobability regions wherein the value of a parameter signal correspondsto a relatively high or low probability of an occurrence of a sensedmoney item of said particular denomination, and a processorconfiguration operable to determine when an occurrence of the parametersignal corresponding to a first money item adopts a first predeterminedvalue relationship, and in response thereto, to compare the value of asubsequent occurrence of the parameter signal corresponding to a secondmoney item with data corresponding to a restricted acceptance range ascompared with the normal acceptance range, and to provide an outputcorresponding to acceptability of the second money item if the secondoccurrence of the parameter signal falls within said restrictedacceptance range, said processor configuration being further operable todetermine when an occurrence of the parameter signal corresponding to afirst money item adopts a second predetermined value relationship with arange of values within said low acceptance probability region for amoney item of a particular denomination, and in response thereto, tocompare the value of a subsequent occurrence of the parameter signalcorresponding to a second money item with data corresponding to aninternal security range within said restricted acceptance range, and toprovide an output corresponding to acceptability of the second moneyitem if the second occurrence of the parameter signal falls outside saidinternal security range.

The processor configuration may be further operable to determine when afirst money item parameter signal adopts said second predetermined valuerelationship, and in response thereto, to compare subsequent occurrencesof the parameter signal with said internal security range, and if afirst number of them correspond to acceptable money items, todiscontinue comparison with the internal security range of values, and,after discontinuing comparison with the internal security range ofvalues, and in response to a subsequent money item parameter signaladopting said second predetermined value relationship, to comparesubsequent occurrences of the parameter signal with said internalsecurity range, and if a second number of them correspond to acceptablemoney items, to discontinue comparison with the internal security rangeof values again, the second number being different from the firstnumber.

The money item acceptor of the second aspect may be arranged such thatthe second number is greater than the first number, and the processormay be operable to increment said first number by a predetermined amountto define said second number. Furthermore a counter may be operable tocount said first number and thereafter to count said second number, andthe processor may be operable to reset the count counted by the counterto a default count value in the event that there is no occurrence of amoney item parameter signal within a predetermined security time period.

The second predetermined value relationship may occur when an occurrenceof the money item parameter signal has a value within said range ofvalues within said low acceptance probability region for a money item ofa particular denomination.

The processor may be operable to compare occurrences of the money itemparameter signal with said internal security range for a firstpredetermined time period following an occurrence of the money itemparameter signal that has said second predetermined value relationship,and then to discontinue comparison with the internal security range, andafter discontinuing comparison with the internal security range tocompare occurrences of the money item parameter signal with saidinternal security range for a second predetermined time period followingan occurrence of the money item parameter signal adopting said secondpredetermined value relationship, and then to discontinue comparisonwith the internal security range again, said second time period beinggreater than the first time period.

In accordance with the invention from a third aspect there is provided amoney item acceptor comprising a signal source to produce a money itemparameter signal as a function of a sensed characteristic of a moneyitem under test, a store to provide data corresponding to an acceptancerange of values of the parameter signal for a money item of a particulardenomination, and a processor configuration operable to determine whenan occurrence of the parameter signal falls within the acceptance range,for accepting the money item, wherein said processor configuration isoperable to provide a focussed rejection window within said acceptancerange and with a disposition dependent on the value of a precedingoccurrence of the parameter signal corresponding to a preceding moneyitem, and to provide an output corresponding to the rejection of themoney item under test if its corresponding parameter signal falls withinthe focussed rejection window. The focussed rejection window may spanthe mean of at least two parameter signals corresponding to precedingmoney items.

The processor may be operable to compare occurrences of the money itemparameter signal with the focussed rejection window until a preselectednumber of successive ones of the occurrences have values falling outsideof the window.

The signal source may be operable to produce a plurality of individualmoney item parameter signals each as a function of a respectivedifferent characteristic of a sensed money item, and the store may beconfigured to provide data for normal acceptance ranges of values, andany focused rejection or other range of values of parameter signals,individually for each of these respective different characteristics.

The invention further includes a corresponding method for detectingfraudulent coins.

An acceptor according to the invention may be configured for use withcoins, banknotes or other money items.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully understood an embodimentthereof will now be described by way of example with reference to theaccompanying drawings in which:

FIG. 1 is a schematic block diagram of a coin acceptor in accordancewith the invention;

FIG. 2 is a schematic block diagram of the circuits of the acceptorshown in FIG. 1;

FIG. 3 a is a distribution curve of coin parameter signals produced bythe acceptor of FIG. 1, illustrating a possible distribution produced bycounterfeit or foreign coins;

FIG. 3 b is a distribution curve of coin parameter signals produced bythe acceptor of FIG. 1, illustrating a possible distribution produced bya set of true coins of a particular denomination and that of a set ofcounterfeit coins;

FIG. 4 is a schematic flow diagram of processing steps carried out bythe microcontroller 11;

FIG. 5 is a schematic flow diagram of further processing steps carriedout by the microcontroller 11 with relation to the upper and lowerinternal security barriers, UISB and LISB;

FIG. 6 is a schematic flow diagram of further processing steps carriedout by the microcontroller 11 with relation to the focused rejectionwindow FRW; and

FIG. 7 is a schematic diagram of a banknote acceptor in accordance withthe invention.

DETAILED DESCRIPTION

Overview of Coin Acceptor

FIG. 1 illustrates the general configuration of an acceptor according tothe invention for use with coins. The coin acceptor is capable ofvalidating a number of coins of different denominations, including bimetcoins, for example the euro coin set and the UK coin set including thebimet £2.00 coin. The acceptor includes a body 1 with a coin run-downpath 2 along which coins under test pass edgewise from an inlet 3through a coin sensing station 4 and then fall towards a gate 5. A testis performed on each coin as it passes through the sensing station 4. Ifthe outcome of the test indicates the presence of a true coin, the gate5 is opened so that the coin can pass to an accept path 6, but otherwisethe gate remains closed and the coin is deflected to a reject path 7.The coin path through the acceptor for a coin 8 is shown schematicallyby dotted line 9.

The coin sensing station 4 includes four coin sensing coil units S1, S2,S3 and S4, which are energised in order to produce an inductive couplingwith the coin. Also, a coil unit PS is provided in the accept path 6,downstream of the gate 5, to act as a credit sensor in order to detectwhether a coin that was determined to be acceptable, has in fact passedinto the accept path 6.

The coils are energised at different frequencies by a drive andinterface circuit 10 shown schematically in FIG. 2. Eddy currents areinduced in the coin under test by the coil units. The differentinductive couplings between the four coils and the coin characterise thecoin substantially uniquely. The drive and interface circuit 10 producescorresponding digital coin parameter data signals x₁, x₂, x₃, x₄, as afunction of the different inductive couplings between the coin and thecoil units S1, S2, S3 and S4. A corresponding signal is produced for thecoil unit PS. The coils S have a small diameter in relation to thediameter of coins under test in order to detect the inductivecharacteristics of individual chordal regions of the coin. Improveddiscrimination can be achieved by making the area A of the coil unit Swhich faces the coin, such as the coil S1, smaller than 72 mm², whichpermits the inductive characteristics of individual regions of thecoin's face to be sensed.

In order to determine coin authenticity, the coin parameter signalsproduced by a coin under test are fed to a microcontroller 11 which iscoupled to a memory 12. The microcontroller 11 processes the coinparameter signals x_(i), -x₄ derived from the coin under test andcompares the outcome with corresponding stored values held in the memory12. The stored values are held in terms of windows having upper andlower value limits. Thus, if the processed data falls within thecorresponding windows associated with a true coin of a particulardenomination, the coin is indicated to be acceptable, but otherwise isrejected. If acceptable, a signal is provided on line 13 to a drivecircuit 14 which operates the gate 5 shown in FIG. 1 so as to allow thecoin to pass to the accept path 6. Otherwise, the gate 5 is not openedand the coin passes to reject path 7.

The microcontroller 11 compares the processed data with a number ofdifferent sets of operating window data appropriate for coins ofdifferent denominations so that the coin acceptor can accept or rejectmore than one coin of a particular currency set. If the coin isaccepted, its passage along the accept path 6 is detected by the postacceptance credit sensor coil unit PS, and the unit 10 passescorresponding data to the microcontroller 11, which in turn provides anoutput on line 15 that indicates the amount of monetary creditattributed to the accepted coin.

The sensor coil units S each include one or more inductor coilsconnected in an individual oscillatory circuit and the coil drive andinterface circuit 10 includes a multiplexer to scan outputs from thecoil units sequentially, so as to provide data to the microcontroller11. Each circuit typically oscillates at a frequency in a range of50-150 kHz and the circuit components are selected so that each sensorcoil S1-S4 has a different natural resonant frequency in order to avoidcross-coupling between them.

As the coin passes the sensor coil unit S1, its impedance is altered bythe presence of the coin over a period of ˜100 milliseconds. As aresult, the amplitude of the oscillations through the coil is modifiedover the period that the coin passes and also the oscillation frequencyis altered. The variation in amplitude and frequency resulting from themodulation produced by the coin is used to produce the coin parametersignals x₁, -x₄ representative of characteristics of the coin.

Processing Circuitry

FIG. 3 a illustrates a bell shaped distribution curve 20 of the valuesof one of the parameters, x₁, produced when a number of coins of thesame denomination are passed through the validator. It can be seen thatmost of the occurrences of the parameter value x₁ occur at a peak valuex_(p) and a generally bell shaped distribution occurs around this peakvalue. The distribution can be determined by passing a number e.g. 100coins of the same denomination through the validator and recording thecorresponding values of x₁. The memory 12 stores data corresponding to awindow of acceptable values of the parameter x₁ for each denomination ofcoin to be accepted by the validator. In FIG. 3 a, one of the windows,referred to herein as a normal acceptance window NAW, is shown,extending between upper and lower window limit values w₁, w₂. The storeddata in memory 12 may comprise the upper and lower window limit valuesw₁, w₂ themselves or may comprise a mean value and a standard deviation,such that the microcontroller 11 can define the window NAW from thestored data as a predetermined number of standard deviations about themean.

The graph of FIG. 3 a can also be considered in a different way. Forcoins of the true denomination that corresponds to the normal acceptancewindow (NAW), the most likely value of parameter x₁ is the peak valuex_(p) and the least likely value occurs at the upper and lower windowlimits w₁, w₂. Whilst it is possible for an acceptable value x_(f) tooccur close to one of the window limits w₁, the probability distributionshown in FIG. 3 a makes it clear that it is unlikely that many suchvalues x_(f) will occur for the true coin concerned. If several valuesx_(f) occur, this is more likely to indicate the presence of afraudulent distribution 23 as shown in dotted outline, with a peak valuecentered on or around x_(f). This property is used in accordance withthe invention to discriminate between true coins and a set of fraudsthat have been manufactured to the same design, or foreign coins, whichproduce coin parameter values x_(f) lying within the normal acceptancewindow NAW. In accordance with the invention, the occurrence of morethan one parameter value x_(f) is considered to be unusual and likely torepresent the occurrence of a fraud. A restricted acceptance window RAWshown in FIG. 3 a is used upon detection of such a situation, as willnow be described.

As shown in FIG. 3 a, upper and lower safety margins LSM, USM aredefined in regions of relatively low probability of an occurrence of aparameter value corresponding to a true coin. It will be understood fromthe distribution curve 20 that it is much more likely for an occurrenceof parameter signal x₁ to occur between the area of relatively highprobability between dotted lines 21, 22 than in the lower and uppersafety margins LSM, USM, where there is a relatively low probability ofoccurrence of a true value. When the microcontroller 11 shown in FIG. 2detects the presence of a value x_(f) in either the LSM or USM, it thenchanges from the normal acceptance window NAW to a restricted acceptancewindow RAW based on data stored in memory 12, which is narrower than thenormal acceptance window, as shown in FIG. 3 a. In practice, the RAW maycorrespond to the region of high probability between the dotted lines21, 22 although different values can be used, which are non-contiguouswith the LSM and USM. If the next, subsequent occurrence of theparameter signal x₁ produced by the next coin under test, occurs in e.g.the USM, close to the previous value x_(f), the next coin will berejected because it lies outside of the restricted acceptance window RAWand is more likely to indicate the presence of a fraudulent coin formingpart of the fraudulent coin distribution 23 than the true coin formingpart of the distribution 20.

When a first coin under test exhibits a parameter signal x_(f) withineither the upper or lower safety margin, USM, LSM of the normalacceptance window NAW, the coin is accepted as a true coin (assumingthat its other detected parameters are satisfactory) but the acceptorthen switches to a restricted acceptance window RAW for subsequentcoins. The occurrence of the first coin with parameter value x_(f) setsa flag which may comprise a counter in the microcontroller 11 thatcounts a coin number parameter n. The acceptor continues to use therestricted acceptance window for a predetermined number of coins n_maxset by the counter, and the flag remains set until a number of coinswith parameter signals x₁ lying within the restricted window RAW occurin succession. The number is dependent upon the distribution of coindata and the probability of a true coin legitimately falling at thelimits of the distribution 20. This will vary from coin to coin buttypically might be six or eight insertions of coin or could be as few asone or as many as twenty.

If another coin produces a value x₁ outside of the restricted acceptancewindow prior to expiry of the count, the flag is reset and the countbegins again. Otherwise, the system reverts to the normal acceptancewindow NAW after n_max coins with parameter signals within the RAW havebeen counted.

However, with the system described so far, there is a risk that afraudster will use true coins in the coin acceptor find out the numbern_max loaded into the counter and then insert a fraudulent cointhereafter, which may be accepted if its coin parameter signal fallswithin the normal acceptance window NAW. According to the invention thecount value n_max is changed e.g. increased, each time the systemreverts to the normal acceptance window so that the fraudster cannotdetermine the current value of n_max that is being used by the counter.The processor sets a security timer routine timer_secure, which sets asecurity time period after which the value of n_max in use is reset to adefault value. It is assumed that after the security time period, thefraudster will have given up and gone away, and that is safe to resetthe value of n_max

Additionally, an upper security barrier USB and a lower security barrierLSB are disposed above and below the upper and lower window limits w₁,w₂ respectively, as shown in FIG. 3 a. If a coin produces a parametersignal x₁ lying within either the upper or lower security barrierregions USB, LSB, the previously described process is carried out andthe acceptor switches from the normal acceptance window NAW to therestricted acceptance window RAW. This process is carried out in orderto reject potentially fraudulent coins that form part of a distributionsuch as the fraudulent distribution 23. For example, it may be possibleto find a coin of a foreign denomination which has a close, similardistribution to the true distribution 20, the foreign coin denominationhaving a distribution 23. The fraudster may attempt to defraud thevalidator by feeding a series of the foreign coins of the samedenomination through the acceptor. With the described arrangementaccording to the invention, although the first foreign coin would beaccepted, those following thereafter would be rejected.

The acceptor may also include a timer which may comprise a routine witha time parameter t run by the microcontrollor 11, that times out after atime period t_max after the restricted acceptance window RAW has beenadopted, and returns the acceptor back to the normal acceptance windowNAW after the time period t_max. The fraudster may insert a fraudulentcoin, get it accepted by the coin acceptor which then switches to use ofthe restricted acceptance window RAW. If the fraudster then gives upafter a few more tries, and goes away, the timer can then time-out intime for an honest user to come and use the acceptor on the basis of thenormal acceptance window NAW. However, there is a risk that thefraudster will ascertain the period t_max after which the system revertsfrom the RAW to the NAW. In accordance with the invention the periodt_max is increased when the system reverts to use of the NAW so as todeter the fraudster. The security timer routine timer secure, may beused to set a security time period after which the value of t_max isreset to a default value. It is assumed that after the security timeperiod, the fraudster will have given up and gone away, and that is safeto reset the value of t_max.

Part of the routine followed by the microcontroller 11 is shown in moredetail in FIG. 4. At step S0, the system is initialised. Theaforementioned counter is set so that its operating parameter n isinitialised i.e. n=0. The default maximum value, n_max (Def), for thiscounter is also set, in this case to 5. Also, the aforementioned timerhas an operating parameter t which can vary from t_max to zero, whichindicates a timed-out condition. At step S0 t is initialised i.e. t=0,and the default maximum value t_max (Def), is set, in this case to 30.Furthermore, the time period after which t_max and n_max, having beenincreased, are reverted back to their default values is initialised i.e.Timer_secure=0.

At step S1, successive values of the parameter signal x₁₁, x₁₂, . . .x_(1N) are shown. These occurrences of the parameter signal are producedin response to the acceptor testing successive coins one after theother. The successive occurrences of the parameter signal are tested oneafter the other by the remainder of the routine as will now beexplained.

At step S2, t_max and n_max are set to their default values, aspreviously mentioned, in the case in which Timer_secure=0. This occursat initialisation of the acceptor and in the case in which the timeassociated with Timer_secure has elapsed and hence any increases ton_max and t_max are reset.

Considering the first occurrence of the parameter signal x₁₁, producedin response to a first coin, at step S3, a test is carried out to see ifthe timer is active. If it is not active, t=0. This means that asufficiently long period of time, t_max, has elapsed since a coin felloutside the restricted acceptance window, indicating that it is safe touse the relatively wide, normal acceptance window NAW.

At step S4, the status of the flag counter is checked. If the flagparameter n=0, this means that the flag is not set and that it is safeto use the normal acceptance window NAW. However, if the flag counter isset whilst the timer is running, it is not safe to use the normalacceptance window because the conditions indicate that a previouslyaccepted coin has triggered the flag counter whilst the timer isrunning. As a result, the value of x₁₁ needs to be compared with therestricted acceptance window RAW. This is carried out at step S5. If thevalue of x₁₁ falls within the restricted acceptance window RAW, the coinis accepted at step S8 but otherwise is rejected at step S7.

As previously mentioned, if the timer or the counter flag are set to 0,it is safe to use the normal acceptance window NAW. This test is carriedout at step S6 and the coin is either accepted or rejected at step S8 orS7.

In addition to comparing the parameter value against either of theacceptance windows, each occurrence of the parameter value is comparedwith the upper and lower safety margins and safety barriers. These testsare performed at steps S9 and S10. If the parameter value signal x₁₁falls within any of the barriers or margins USB, USM, LSB, LSM, thisindicates that the aforementioned flag needs to be set and that thetimer t should be set running. These activities are carried out at stepS12, at which the count parameter n is set to a predetermined maximumvalue n_max. It will be understood that n_max is an integer numbercorresponding to the number of successive coins which need to be foundto be true when using the relatively narrow restricted acceptance windowRAW in order to revert to the normal acceptance window. The value of thetimer interval t is set to t_max which corresponds to the period of timefor which the timer will run until reaching a value t=0. This, thereforesets the time after which the acceptor will recover and switch back touse the normal acceptance window NAW after a period of using therestricted acceptance window RAW (step S3).

If the value of the parameter signal x₁₁ does not fall within any of themargins or barriers tested by step S9, S10, this indicates that theparameter signal x₁₁, on the assumption that the coin has been accepted,falls within the restricted acceptance window RAW. In this situation,the counter parameter n needs to be decremented, if it is not alreadyzero. This occurs at step S11 in addition to other steps which aredescribed below.

When the count parameter n reaches the value 1, the values of n_max andt_max are increased so that the next fraudulent attempt to occur has anincreased number of true insertions and time to have elapsed beforereverting to normal acceptance window. The parameters n_max and t_maxare therefore increased, for example, by 2 and 20% respectively at steps11. Additionally, the Timer_secure timer is set to a value TS_max. Oncethis time TS_max has elapsed, n_max and t_max are returned to theirrespective default values n_max(def), and t_max(def), as previouslydescribed, at step S2.

Considering the situation where the first occurrence of the coinparameter signal x₁₁ falls within the upper safety margin USM. In thissituation, t=0 and n=0 so that the routine passes through steps S3 andS4 to step S6 at which the value is compared with the normal acceptancewindow NAW. The value of x₁₁ falls within the window NAW and hence thecoin is accepted at step S8.

Additionally, the value of x₁₁ is found to be within the upper safetymargin USM, at step S9. As a result, the flag counter parameter n is setto n_max and the timer parameter t is set to t_max at step S12.

When a second coin is entered a second occurrence of the coin parametersignal x₁ is produced, namely x₁₂. At step S3, the timer is now set tot≠0 and so the process moves to step S4. The parameter n≠0 and so thevalue of x₁₂ is compared with the restricted acceptance window RAW atstep S5. The value is either accepted or rejected. Assuming it isaccepted, and falls outside of the margins and barriers tested at stepS9 and S10, the counter parameter n is decremented at step S11. Thetimer t is running during this time towards zero.

The process continues with the subsequent occurrences of the parameterx₁ so that coins that fall within the RAW decrement the counter flaguntil the timer t=0 or the counter flag n=0. The acceptor then revertsto the use of the normal acceptance window NAW. When the counter flag nreached 1 however, the values of n_max and t_max were increased, at steps11, becoming 7 and 36 respectively. The Timer_secure timer was also setto TS_max. Should another coin fall outside the restricted acceptancewindow within the time TS_max, the n_max and t_max values applied to nand t respectively at s12 would now be 7 and 36 respectively. OnceTS_max has elapsed these would be reverted to the default values at S2of 5 and 30 respectively.

In order that the invention may be more fully understood, a descriptionof the processes carried out by the microcontroller in response to anumber of coin insertions by a fraudster will now be given, withreference to FIG. 4.

Considering the situation involving the first use of the coin acceptor.The system is primarily initialised at step S0. The default values n_max(Def) and t_max (Def) are set to 5 and 30 respectively and Timer_secure,n and t are each set to 0. A first fraudulent coin is then inserted andthe parameter value x₁₁ determined and sent to the processor as part ofstep S1. This triggers the system to move to step S2 at which, becausetimer_secure=0, n_max is set to n_max (Def) i.e. 5, and t_max is set tot_max (Def) i.e. 30.

The query at step S3 returns a positive outcome as t=0 and the firstfraudulent coin parameter is thus compared to the normal acceptancewindow at step S5. The first fraudulent coin parameter may or may notfall inside the NAW, but in this case it will be assumed that it does.Accordingly, the coin will be accepted at step S8.

The queries at steps S9 and S10 are triggered essentially simultaneouslyto that of S3. Assuming the fraudulent coin parameter x₁₁ falls outsidethe restricted acceptance window, which is most likely to be the case,x₁₁ will hence have fallen within the upper or lower security margins,USM or LSM. Step S10 thus returns a positive value and n and t are setto n_max and t_max at step S12, i.e. 5 and 30 respectively.

The fraudster has now had one fraudulent coin accepted. The fraudsterhowever knows from previous fraudulent attempts on other coin acceptorsthat the restricted acceptance window will apply until a certain numberof true coins have been inserted. To determine this number he insertsprogressively larger groups of true coins in succession, each timefollowed by a fraudulent coin and waits until a fraudulent coin isaccepted. Referring to FIG. 4, the first true coin would result in thefollowing processing steps.

The true coin is inserted and the parameter x₁₂ determined and sent tothe processor at step S1. The IF statement of step S2 is again true astimer_secure=0 and so n_max and t_max are again set to their defaultvalues. The queries of steps S3 and S4 return negative responses as t≠0and n≠0. This results in a comparison of the true coin parameter x₁₂with the restricted acceptance window. The parameter x₁₂ falls insidethe RAW, as the majority of true coins would, and so it is accepted.Accordingly the parameter x₁₂ does not fall within USB, LSB, LSM or USM.Steps S9 and S10 return negative responses and the processor moves tostep S11. The variable n=5 is greater than 0 and so n is decremented ton=4. The next IF statement of S11 is untrue as n≠1 and so the processesstop and the system awaits the next coin insertion.

The fraudster might now insert 4 more true coins, guessing that then_max value for the machine is 5. Each would result in the sameprocessing steps to be taken as the first true coin described above,with n decrementing each time until it reaches 0. However, of the 5 truecoin insertions, the 4^(th) true coin would also trigger some addedevents at step S11. When the processing of the fourth coin parameterreaches step S11, n is decremented from n=2 to n=1. This then results inthe second IF statement of step S11 being true. Accordingly n_maxbecomes n_max+2, i.e. 7, and t_max becomes 1.2 t_max i.e. 36.Timer_secure is then set to TS_max, the value of which is not specifiedin FIG. 4, but could be set to a value larger than t_max.

Now, having inserted 5 true coins, the fraudster may decide to attemptanother fraudulent coin. The fraudulent coin is inserted and theparameter x₁₇ determined and sent to the processor at step S1. The IFstatement of step S2 is false as timer_secure ≠0 and so n_max and t_maxremain at the increased values 7 and 36 respectively. The query of stepS3 may return a negative response as it could still be at t>0, however,step S4 will now return a positive response because n=0. This results ina comparison of the fraudulent coin parameter x₁₇ with the normalacceptance window. The parameter x₁₇, although coming from a fraudulentcoin, could fall inside this window in which case it would be acceptedat step S8. The parameter x₁₇ is likely to fall within LSM or USM and sostep S10 would accordingly return a positive response and the processorwould then move to step S12. At step S12, n is set to n_max and t tot_max, which are the increased values 7 and 36.

The fraudster, using his previously gained knowledge of this coinacceptor, would now insert a further 5 true coins followed by afraudulent coin expecting this combination, as before, to be accepted.However, as n has now been set to the increased value 7, the restrictedacceptance window would still be in operation and the fraudulent coin istherefore most likely to be rejected. This would confuse the fraudster,who may now decide to go away and wait until the normal time t haslapsed, after which, from prior experience, he may know use of thenormal acceptance window will be resumed. However, this time has alsobeen increased and so the fraudsters next fraudulent coin would also berejected. Furthermore, this fraudulent attempt would increase furtherthe values of n_max and t_max. By the time the timer_secure time haslapsed, the fraudster is very likely to have given up with trying tocheat this coin acceptor, and at this stage the use of the defaultvalues of n_max and t_max can be resumed.

The previously described process thus relates to one of the coinparameter signals x_(1N). However, as previously explained, fourdifferent coin parameter signals x₁-x₄ are produced in this example andin fact, in practice, up to fourteen different individual parametersignals may be processed. The routine performed according to FIG. 4 maybe carried out for each individual coin parameter signal with eachhaving its own normal acceptance window and restricted acceptancewindow, controlled as previously described, with each parameter signalbeing processed independently of the others. Alternatively, to simplifythe processing, the occurrence of one parameter signal falling withinits respective USB, LSB, LSM or USM may trigger the use of an individualrestricted acceptance window for all of the coin parameter signalsconcurrently.

Other modifications are possible. In the routine shown in FIG. 4, thecounter flag is clocked downwardly from a first predetermined numbern_max. Typically n_max is in a range of 4 to 20 inclusive. Whilst n≠0the restricted acceptance window RAW is used (step S4). However, whenn=0 i.e. when 4 to 20 true coins have been detected, the normal windowNAW is used. The occurrence of a single fraudulent coin will thenre-trigger the use of the RAW (steps S9, S10 and S12). However, ifdesired a different pre-selected number p of occurrences of fraudulentcoin could be used to reset n=n_max and thereby re-trigger the use ofthe RAW. The pre-selected number p of occurrences of fraudulent coin isselected to be less than the predetermined number n to thereby improvethe sensitivity of the system. Preferably the number p is 1 as describedwith reference to FIG. 4 to maximise the sensitivity to fraudulentcoins, although a larger value of p may in some instances be desirableto provide system damping.

In another modification, the routine may switch from the normalacceptance window NAW to the RAW in response to a coin parameter signalfalling within a very narrow portion of the NAW itself, which maysignify a fraudulent coin in certain circumstances.

FIG. 3 b, similar to FIG. 3 a, illustrates a bell-shaped distributioncurve 20 of the values of one of the parameters, x₁, produced when anumber of coins of the same denomination are passed through thevalidator. Again, most of the occurrences of the parameter value x₁occur at a peak value x_(P). The normal and restricted acceptancewindows, NAW and RAW, are also illustrated. An upper and lower internalsecurity band, UISB and LISB have been introduced inside the restrictedacceptance window RAW. The curve R_(F) represents the distribution ofparameter values x₁ produced by many counterfeit coins passed throughthe validator. This has a relatively sharp peak which lies within theRAW. If several consecutive parameter values x_(F) occur within a smallnumber of coin insertions and are within one of these bands UISB orLISB, this is more likely to indicate the presence of a fraudulent coinsuch as those belonging to a distribution such as R_(F), with a peakcentred in one of these bands. For this reason, following the detectionof a parameter within either of the internal security bands UISB orLISB, further coins with parameters within these bands will be rejecteduntil a certain number n2_max of coins have been inserted which do notfall within these bands. A counter with count value n2 may be loadedwith the value n2_max and decremented following each coin parameterwhich falls outside UISB and LISB. Once the counter reaches 0,acceptance within UISB and LISB can be resumed.

There is a risk that a fraudster will use true coins in the coinacceptor which do not fall within UISB or LISB, find out the numbern2_max loaded into the counter n2, and then insert a fraudulent cointhereafter, which may now be accepted if its coin parameter signal fallswithin an internal security band. According to the invention the countvalue n2_max is changed e.g. increased, each time the system returns toacceptance within UISB and LISB so that the fraudster cannot determinethe current value of n2_max that is being used by the counter. Theprocessor sets a security timer routine timer_secure2, which sets asecurity time period after which the value of n2_max in use is reset toa default value. It is assumed that after the security time period, thefraudster will have given up and gone away, and that is safe to resetthe value of n2_max to a default value n2_max (Def).

The acceptor may also include a timer which may comprise a routine witha time parameter t2 run by the microcontrollor 11, that times out aftera time period t2_max after acceptance within UISB and LISB has beendisabled, and the acceptor is then reverted back to enable acceptance.The fraudster may insert a fraudulent coin falling within UISB or LISB,get it accepted by the coin acceptor which then disables UISB and LISB.If the fraudster then gives up after a few more tries, and goes away,the timer can then time-out in time for an honest user to come and usethe acceptor with resumed use of UISB and LISB. However, there is a riskthat the fraudster will ascertain the period t2_max after which thesystem reverts from disabled to enabled internal security bands. Inaccordance with the invention the period t2_max is increased when thesystem reverts to enabled acceptance within UISB and LISB so as to deterthe fraudster. The security timer routine timer_secure2, may be used toset a security time period after which the value of t2_max is reset to adefault value. It is assumed that after the security time period, thefraudster will have given up and gone away, and that is safe to resetthe value of t2_max to a default value t2_max (Def).

An example of the part of the routine followed by the microcontroller 11with respect to the upper and lower internal security bands is shown inmore detail in FIG. 5. This routine may be followed by themicrocontroller in conjunction with the routine of FIG. 4 in order thatthe UISB and LISB aspect is provided as an additional security featureto those features already existing in the normal money item acceptor.

At step S13, the system is initialised. The aforementioned counter isset so that its operating parameter n2 is initialised i.e. n2=0. Thedefault maximum value, n2_max (Def), for this counter is also set, inthis case to 5. Also, the aforementioned timer has an operatingparameter t2 which can vary from t2_max to zero, which indicates atimed-out condition. At step S13 t2 is initialised i.e. t2=0, and thedefault maximum value t2_max (Def) is set, in this case to 30.Furthermore, the time period after which t2_max and n2_max, having beenincreased, are reverted back to their default values is initialised i.e.timer_secure2=0.

At step S14, successive values of the parameter signal x₁₁, x₁₂, . . .x_(1N) are shown. These occurrences of the parameter signal are producedin response to the acceptor testing successive coins one after theother. The successive occurrences of the parameter signal are tested oneafter the other by the remainder of the routine as will now beexplained.

At step S15, t2_max and n2_max are set to their default values, aspreviously mentioned, in the case in which timer_secure2=0. This occursat initialisation of the acceptor and in the case in which the timeassociated with timer_secure2 has elapsed and hence any increases ton2_max and t2_max are reset.

Considering the first occurrence of the parameter signal x₁₁, producedin response to a first coin. At step S20, a test is carried out to seeif the timer is active. If it is not active, t2=0. This means that asufficiently long period of time, t2_max, has elapsed since a coin fellin the UISB or LISB, indicating that it is safe to enable acceptancewithin these bands. This part of the routine would then finish and themicrocontroller would move on to another routine, as shown by thedownward arrow at the bottom of FIG. 5.

In the case where t2≠0, at step S21, the status of the flag counter n2is checked. If the flag parameter n2=0, this means that the flag is notset and that it may be safe to enable acceptance within UISB and LISB.However, if the flag counter is set whilst the timer is running, it isnot safe to enable acceptance within UISB and LISB because theconditions indicate that a previously accepted coin has triggered theflag counter whilst the timer is running. As a result, the coinassociated with the value x₁₁ will be rejected at S23 should it fallwithin UISB or LISB, the test for which is carried out at step S22.

Each occurrence of the parameter value is compared with the upper andlower internal security bands again at steps S16 and S17. If theparameter value signal x₁₁ falls within LISB or UISB, this indicatesthat the aforementioned flag n2 needs to be set and that the timer t2should be set running. These activities are carried out at step S19, atwhich the count parameter n2 is set to a predetermined maximum valuen2_max. It will be understood that n2_max is an integer numbercorresponding to the number of successive coin parameters which need tobe found to be outside UISB and LISB before acceptance within UISB andLISB can be resumed. The value of the timer interval t2 is set to t2_maxwhich corresponds to the period of time for which the timer will rununtil reaching a value t2=0. This, therefore sets the time after whichthe acceptor will recover and switch back to acceptance within UISB andLISB (step S20).

If the value of the parameter signal x₁₁ does not fall within eitherUISB or LISB as tested by steps S16 and S17, this indicates that theparameter signal x₁₁, is not likely to be part of a fraudulent set withparameter values in the outer edge of the RAW. In this situation, thecounter parameter n2 needs to be decremented, if it is not already zero.This occurs at step S18 in addition to other steps which are describedbelow.

When the count parameter n2 reaches the value 1, the values of n2_maxand t2_max are increased so that the next fraudulent attempt to occurhas an increased number of true insertions (those falling outside UISBand LISB) and time to have elapsed before reverting to acceptance withinUISB and LISB. The parameters n2_max and t2_max are therefore increased,for example, by 2 and 20% respectively at step S18. Additionally, theTimer_secure2 timer is set to a value TS2_max. Once this time TS2_maxhas elapsed, n2_max and t2_max are returned to their respective defaultvalues n2_max(def), and t2_max(def), as previously described, at stepS15.

Considering the situation where the system is initialised at step S13,and the first occurrence of the coin parameter signal x₁₁ occurs at S14.At S15, Timer_secure2=0 is true, and hence n2_max and t2_max are set totheir default conditions, i.e. 5 and 30 respectively. Assuming x₁₁ fallswithin the upper internal security band UISB. Firstly, the routine maypass to S20. Here, the test t2=0 returns a true response, so thisparticular routine ends.

Additionally, the value of x₁₁ is tested at S16 and S17. The parameteris found to be within the upper internal security band UISB, at stepS17. As a result, the flag counter parameter n2 is set to n2_max and thetimer parameter t2 is set to t2_max at step S19.

When a second coin is entered a second occurrence of the coin parametersignal x₁ is produced, namely x₁₂. At step S20, the timer is now set tot2≠0 and so the process moves to step S21. The parameter n2≠0 and so thevalue of x₁₂ is compared with the bands UISB and LISB at S22. The valueis rejected should the parameter fall within either of these bands.Assuming it is accepted, and therefore also falls outside of the bandstested at step S16 and S17, the counter parameter n2 is decremented atstep S18. The timer t2 is running during this time towards zero.

The process continues with the subsequent occurrences of the parameterx₁ so that coins that fall outside the UISB or LISB bands decrement thecounter flag until the timer t2=0 or the counter flag n2=0. In themeantime, any parameters falling within UISB or LISB will reset n2 andt2 to n2_max and t2_max at S19. When n2=0 or t2=0, the acceptor thenreverts to acceptance within UISB and LISB. When the counter flag n2reached 1 however, the values of n2_max and t2_max were increased, atstep s18, becoming 7 and 36 respectively. The Timer_secure2 timer wasalso set to TS2_max. Should another coin fall inside UISB or LISB withinthe time TS2_max, the n2_max and t2_max values applied to n2 and t2respectively at s19 would now be 7 and 36 respectively. Once TS2_max haselapsed and Timer_secure=0, these would be reverted to the defaultvalues at S15 of 5 and 30 respectively.

The previously described process thus relates to one of the coinparameter signals x_(1N). However, as previously explained, fourdifferent coin parameter signals x₁-x₄ are produced in this example andin fact, in practice, up to fourteen different individual parametersignals may be processed. The routine performed according to FIG. 5 maybe carried out for each individual coin parameter signal with eachhaving its own upper and lower internal security bands, controlled aspreviously described, with each parameter signal being processedindependently of the others. Alternatively, to simplify the processing,the occurrence of one parameter signal falling within its respectiveUISB or LISB may disable acceptance within the individual internalsecurity bands for all of the coin parameter signals concurrently.

Other modifications are possible. In the routine shown in FIG. 5, thecounter flag n2 is clocked downwardly from a first predetermined numbern2_max. Typically n2_max is in a range of 4 to 20 inclusive. Whilstn2≠0, parameters falling within UISB and LISB are rejected (step S21).However, when n2=0 i.e. when 4 to 20 true coins have been detected,acceptance within UISB and LISB is resumed. The occurrence of a singlefraudulent coin falling within UISB or LISB will then re-triggerrejection within UISB and LISB (steps S16, S17 and S19). However, ifdesired a different pre-selected number p of occurrences of fraudulentcoin could be used to reset n2=n2_max and thereby re-trigger acceptancewithin UISB and LISB. The pre-selected number p of occurrences offraudulent coin is selected to be less than the predetermined number n2to thereby improve the sensitivity of the system. Preferably the numberp is 1 as described with reference to FIG. 5 to maximise the sensitivityto fraudulent coins, although a larger value of p may in some instancesbe desirable to provide system damping.

In addition to the enhanced security features of the USM, LSM, USB, LSB,UISB and LISB, a further system is applied to minimise to risk of fraudfrom counterfeit coins. As previously explained, the curve R_(F) shownin FIG. 3 b, represents the distribution of parameter values X₁ producedby many counterfeit coins passed through the validator. This has arelatively sharp peak which lies within the RAW. If several consecutiveparameter values x_(F) occur within a small number of coin insertionsand have a small margin separating them, this is more likely to indicatethe presence of a fraudulent coin such as those belonging to R_(F). Inaccordance with the invention, a focused rejection window (FRW), asshown in FIG. 3 b, is applied in addition to the normal acceptancewindow upon detection of such a situation, as will now be described.

The focused rejection window, FRW, is used in accordance with theinvention to discriminate between true coins and a set of frauds thathave been manufactured to the same design and which produce coinparameter values R_(F) lying within the restricted acceptance windowRAW. The FRW is calculated to be a relatively narrow window compared tothe RAW. In a preferred embodiment of this invention, the range of thefocused rejection window is centred at the mean of the two parametersignals, and has limits at, for instance, plus and minus 5% of the mean.The occurrence of the first coin with a parameter value within a smallmargin of a preceding parameter relating to a preceding coin sets a flagwhich may comprise a counter (with operating parameter n_(FRW)) in themicrocontroller 11. The acceptor continues to use the FRW for apredetermined number of coin insertions set by the counter, and the flagremains set until a number of coins with parameter signals x₁ lyingoutside the FRW occur in succession. The number is dependent upon thedistribution of coin data and the probability of a true coinlegitimately falling within the FRW. This will vary from coin to coinbut typically might be six or eight insertions of coin or could be asfew as one or as many as twenty.

An example of the part of the routine followed by the microcontroller 11with respect to the focused rejection window is shown in more detail inFIG. 6.

This routine may be followed in conjunction with the routine of FIG. 4,or the routine of FIG. 5, or in conjunction with the routines of FIGS. 4and 5. In this manner, the FRW aspect is provided as an additionalsecurity feature to those features already existing in the money itemacceptor.

Referring to FIG. 6, at step S24, the system is initialised. Theaforementioned counter is set so that its operating parameter n_(FRW) isinitialised i.e. n_(FRW)=0. This counter counts the number of successivecoin insertions not falling inside the FRW, which need to take placebefore use of the FRW is ended.

At step S25, successive values of the parameter signal x₁₁, x₁₂, . . .x_(1N) are shown. These occurrences of the parameter signal are producedin response to the acceptor testing N successive coins one after theother. The successive occurrences of the parameter signal are tested oneafter the other by the remainder of the routine as will now beexplained.

At step S26, the microcontroller determines whether a focused rejectionwindow is in operation by determining the status of the count flagn_(FRW). If this has the value n_(FRW)>0, i.e. the focused rejectionwindow is in operation, then the parameter value x_(1N) is compared tothe focused rejection window at S27. Should the parameter value fallwithin FRW the coin is rejected at S29 and the counter is reset at S33to a preset maximum value n_(FRWmax).

If, at S26, the value n_(FRW)=0, this suggests that a focused rejectionwindow is not in operation and the microcontroller determines whetherthe parameter falls within the restricted acceptance window RAW at stepS28. If this is the case, at S30 it is decided whether or not a new FRWneeds to be implemented. In the example of the figure the differencebetween the coin parameter value x₁₂ associated with coin 2 and theparameter value x₁₁ associated with coin 1 is determined. However, inanother preferred embodiment of this invention this difference would bedetermined between the parameter associated with the current coin andwith a certain number of preceding coins in addition to simply thedirectly preceding coin as shown. Should this difference be less thanthe small margin E, the FRW is created at S32. In this example the FRWis determined to be a range centred at the mean of x₁₁ and x₁₂, althoughthis could be calculated as a larger or smaller range, and with anoffset from the mean if desired. At S33 the counter n_(FRW) is set ton_(FRWmax).

Should a coin parameter at S30 not fall within the small margin E of apreceding parameter signal, or if the parameter at S28 does not fallinside the RAW, the counter n_(FRW) is decremented at S31.

Considering the situation where a second coin is inserted into theacceptor which has a coin parameter signal x₁₂ which falls within themargin E of the first occurrence of the coin parameter signal x₁₁. Inthis situation, n_(FRW)=0 so that the routine passes to step S28 atwhich the value is compared with the restricted acceptance window RAW.If the value of x₁₂ falls within the window then the margin ofdifference between x₁₁ and x₁₂ is determined at S30. Assuming this issmaller than E, the FRW is calculated at S32 and at S33 the flag counterparameter n_(FRW) is set to n_(FRWmax).

When a third coin is entered a third occurrence of the coin parametersignal x₁ is produced, namely x₁₃. At step S26, the counter is now setto n_(FRW)≠0 and so the process moves to step S27. If the parameterfalls within the FRW the coin is rejected at S29 and the counter resetat S33. If the parameter does not fall within the FRW the coin is testedas a normal coin from S28, leading to the counter being decremented or anew FRW implemented if necessary according to the result of step S30.

The process continues with the subsequent occurrences of the parameterx₁ until the counter flag n_(FRW)=0, at which point the use of the FRWis ended.

In order that the invention may be more fully understood, a descriptionof the processes carried out by the microcontroller in response to anumber of coin insertions by a fraudster will now be given, withreference to FIG. 6.

Considering the situation involving the first use of the coin acceptor.The system is primarily initialised at step S24. This may involve thecounter n_(FRW) being set to n_(FRW)=0, as shown in FIG. 6. The firstfraudulent coin is inserted by the fraudster, and a parameter value x₁₁is produced and sent to the processor at step S25. The receipt of thisparameter signal triggers the processor to move to step S26 and hencequestion whether a FRW is currently being used. As n_(FRW)=0, the queryof S26 returns a positive outcome and the processor moves to step S28.The fraudulent coin that was inserted by the fraudster is assumed tobelong to the distribution R_(F) which is within the restrictedacceptance window RAW and accordingly the query S28 returns a positiveoutcome and the processor moves to step S30. At S30 the parameter x₁₁would be compared to a parameter associated with a preceding coininsertion. However, as no preceding coins exist the system would move toS31. The IF statement of S31 is false as n_(FRW)=0 and hence theprocessor routine stops and the system awaits the next coin entry.

The fraudster may now insert a second fraudulent coin of thedistribution R_(F). At S25 the processor receives the parameter x₁₂associated with this fraudulent coin. The query at step S26 returns apositive outcome because n_(FRW)=0, as does the query of S28 because x₁₂is within the RAW. At step S30 the difference between x₁₂ and x₁₁ isdetermined and compared to a value E. This value E could be set to beequal to half the FRW width, as is shown in FIG. 6, or another valuedependent on the probability associated with having two parametersseparated by the value E and produced by true coins. Assuming x₁₂ fallswithin a separation of E from x₁₁, the query of S30 returns a positiveoutcome and the processor moves to step S32. At S32 the FRW is created,being, in this example, set to the mean of the first two parametersignals x₁₁ and x₁₂ and spanning the range E to either side of thismean. At S33 the counter n_(FRW) is set to a predetermined maximumvalue, n_(FRWmax), which may be between 4 and 20, and the routine thenstops and awaits the next coin entry.

A third fraudulent coin inserted by the fraudster of the distributionR_(F) results in, at step S25, the processor receiving the parameter x₁₃associated with this fraudulent coin. The query at step S26 now returnsa negative response because n_(FRW)≠0. The query of step S27 checkswhether the parameter x₁₃ is within the FRW. As x₁₃ belongs to thedistribution R_(F) this is likely to be true and therefore a positiveresponse is returned. This results in the coin being rejected at stepS29 and the counter value n_(FRW) being reset to n_(FRWmax) at step S33.Any further fraudulent coins of the distribution R_(F) will be rejectedin a similar way until a number n_(FRWmax) of successive coins withparameter signals falling outside this FRW have been inserted.

Although FIG. 6 refers to the use of one focussed rejection window, FRW,and one count parameter n_(FRW), there could equally be multiplefocussed rejection windows implemented, each having associated countparameters, so that the system could tackle situations involving morethan one fraudulent coin set such as R_(F).

The previously described process thus relates to one of the coinparameter signals x_(1N). However, as previously explained, fourdifferent coin parameter signals x₁-x₄ are produced in this example andin fact, in practice, up to fourteen different individual parametersignals may be processed. The routine performed according to FIG. 6 maybe carried out for each individual coin parameter signal with eachhaving its own restricted acceptance window and focused rejectionwindow, controlled as previously described, with each parameter signalbeing processed independently of the others.

Other modifications are possible. In the routine shown in FIG. 6, thecounter flag is clocked downwardly from a first predetermined numbern_(FRWmax). Typically n_(FRWmax) is in a range of 4 to 20 inclusive.Whilst n_(FRW)≠0 the focused acceptance window FRW is used (step S3).However, when n_(FRW)=0 i.e. when 4 to 20 true coins have been detected,the use of the FRW is removed. The occurrence of a single fraudulentcoin with a parameter signal which falls within a small margin of apreceding coin's parameter signal will then re-trigger the use of theFRW (steps S30). However, if desired a different pre-selected number pof occurrences of fraudulent coin could be used to resetn_(FRW)=n_(FRWmax) and thereby re-trigger the use of the FRW. Thepre-selected number p of occurrences of fraudulent coin is selected tobe less than the predetermined number n_(FRW) to thereby improve thesensitivity of the system. Preferably the number p is 1 as describedwith reference to FIG. 6 to maximise the sensitivity to fraudulentcoins, although a larger value of p may in some instances be desirableto provide system damping.

Banknote Acceptor

The previously described routine is also applicable to banknoteacceptors and an example is shown in FIG. 6. A banknote 30 to be testedis inserted between driven rollers 31, 32 so as to pass over a sensingplaten 33 over which a series of banknote sensors are disposed. In thisexample, four sensors S1, S2, S3 and S4 are shown schematically. Thesensors may include optical sensors for sensing the length, width orthickness of the banknote, sensors for detecting reflected light fromthe banknote in order to analyse the spectral response. Alternatively,the light may be sensed in transmission through the banknote. One ormore individual predetermined parts of the banknote may be measured.Also, the presence of magnetic printing ink may be detected as describedin U.S. Pat. No. 4,864,238. The sensors S1-S4 are driven and processedby drive and interface circuitry 10 to produce individual parametersignals x₁, x₂, x₃, x₄. These parameter signals are similar to thecorresponding signals described with reference to FIGS. 1 and 2 for thecoin acceptor although indicative of different parameters relating to abanknote. The resulting signals thus can be processed according to thepreviously described routine. The parameter signals are passed to amicrocontroller 11 connected to a memory 12 that contains stored windowvalues. The parameter signals are compared with stored windowscorresponding to acceptable banknotes in the manner previously describedwith reference to FIGS. 4, 5 and 6, and upon detection of an acceptablebanknote, an output is provided on line 13 to a gate driver 14 whichoperates a gate 34. If the banknote is found to be acceptable, it ispassed to a store 35 but otherwise is fed into a reject path 36 andpasses out of the acceptor.

Thus, in accordance with the invention, the banknote acceptor isprovided with increased security to discriminate against a fraudsterinserting a series of fraudulent banknotes all made according to thesame design, which individually would fall within the normal acceptancewindow for an acceptable denomination of banknote.

Whilst the invention has been described by way of example in relation toa coin acceptor and a bank note acceptor it will be understood that itis applicable to other money items such as tokens which are sometimesused instead of coins and other sheet members which have an attributablemoney value including, but not limited to, credit and debit cards.

1. A money item acceptor comprising: a signal source to produce a moneyitem parameter signal as a function of a sensed characteristic of amoney item, a store to provide data corresponding to a normal acceptancerange of values of the parameter signal for a money item of a particulardenomination, having high and low acceptance probability regions,wherein the value of the parameter signal corresponds to a high or lowprobability of an occurrence of a sensed money item of said particulardenomination, and a processor configuration operable to control a gatefor directing money items towards an accept path or a reject path, theprocessor configuration further configured to determine when anoccurrence of the parameter signal corresponding to a first money itemfalls outside of the normal acceptance range, and to provide an outputto the gate to direct the first money item towards the reject path, todetermine when an occurrence of the parameter signal corresponding tothe first money item falls within the normal acceptance range and withinthe low acceptance probability region of the normal acceptance range,and to provide an output to the gate to direct the first money itemtowards the accept path, and to compare the value of the occurrence ofthe parameter signal corresponding to the second money item with datacorresponding to the restricted acceptance range, and to provide anoutput to the gate to direct the second money item towards the acceptpath when the occurrence of the parameter signal corresponding to thesecond money item falls within said restricted acceptance range, and toprovide an output to the gate to direct the second money item towardsthe reject path when the occurrence of the parameter signalcorresponding to the second money item falls outside said restrictedacceptance range, said processor configuration being further configuredto determine when an occurrence of the parameter signal corresponding tothe first money item falls outside of an internal security range ofvalues within the high acceptance probability region and within thenormal acceptance range, and to provide an output to the gate to directthe first money item towards the accept path, said processorconfiguration being further configured to determine when an occurrenceof the parameter signal corresponding to the first money item fallswithin the internal security range of values within said high acceptanceprobability region of the normal acceptance range, and to provide anoutput to the gate to direct the first money item towards the acceptpath, and to compare the value of the parameter signal corresponding tothe second money item with data corresponding to said internal securityrange, and to provide an output to the gate to direct the second moneyitem toward the accept path when the occurrence of the parameter signalcorresponding to the second money item falls outside said internalsecurity range and within said high acceptance probability region, andto provide an output to the gate to direct the second money item towardsthe reject path when the occurrence of the parameter signalcorresponding to the second money item falls within said internalsecurity range or outside said high acceptance probability region.
 2. Anacceptor according to claim 1 wherein, said processor configuration isfurther configured, in response to said first money item parametersignal falling within the internal security range of values to comparesubsequent occurrences of the parameter signal with said internalsecurity range, and when a first number of money items are accepted, todiscontinue comparison with the internal security range of values, and,after discontinuing comparison with the internal security range ofvalues, and in response to a subsequent money item parameter signalfalling within the internal security range of values, to comparesubsequent occurrences of the parameter signal with said internalsecurity range, and when a second number of money items are accepted, todiscontinue comparison with the internal security range of values again,the second number being different from the first number.
 3. An acceptoraccording to claim 2 wherein the second number is greater than the firstnumber.
 4. An acceptor according to claim 2 wherein the processor isconfigured to increment said first number by a predetermined amount todefine said second number.
 5. An acceptor according to claim 2comprising a counter configured to count said first number andthereafter to count said second number.
 6. An acceptor according toclaim 5 wherein the processor configuration is configured to reset thecount counted by the counter to a default count value in the event thatthere is no occurrence of a money item parameter signal within apredetermined security time period.
 7. An acceptor according to claim 1wherein the processor configuration is configured to compare occurrencesof the money item parameter signal with said internal security range fora first predetermined time period following an occurrence of the moneyitem parameter signal that falls within said internal security range,and then to discontinue comparison with the internal security range. 8.An acceptor according to claim 7 wherein the processor configuration isconfigured, after discontinuing comparison with the internal securityrange, to compare occurrences of the money item parameter signal withsaid internal security range for a second predetermined time periodfollowing an occurrence of the money item parameter signal fallingwithin said internal security range, and then to discontinuingcomparison with the internal security range, said second time periodbeing greater than the first time period.
 9. An acceptor according toclaim 8 wherein the processor is configured to define the second timeperiod as a predetermined percentage increase of the first time period.10. An acceptor according to claim 8 including a timer configured totime said first time period and said second time period.
 11. An acceptoraccording to claim 8 wherein the processor configuration is configuredto reset the time period timed by the timer to a default value in theevent that there is no occurrence of a money item parameter signalwithin a predetermined security time period.